Roller cams/tappets and flat tappet cams are incompatible to mix. You might get away with it short term, but eventually it will destroy the cam and/or the lifters. And maybe take the rest of the engine with them.
Why you ask?
A flat tappet isn't actually "flat" it just appears so because the radius of the curve is so large*. But if you put them on a sheet of glass, you will find they rock slightly versus sit solid. The corresponding cam has the lobs ground with a very subtle taper. The combination of the taper and the curve results in the tappet rotating in the lifter bore. That way the contact position is constantly changing on both the lifter and the cam, so wear is very, very slow. The larger radius and subtle taper allow the the oil film to distrubute the load
In contrast, as roller cam is machined flat to match the flat of the roller in the roller tappet therefore maximizing the contact surface.
If you put flat tappets with a roller cam**, the contact postion on the tapper never changes and after very short period the tappet wears and metal shavings fall into oil causing expensive damage.
If you put roller tappets on a "flat" tappet cam, only the edge of the cam is in contact with the roller and after short period, the tappet wears the cam or vice versa and again metal shavings fall into oil causing expensive damage.
* This assumes your cam manufacturer knows what they are doing. Failing to machine taper/radius is most common issue on reground cams/lifters, 2nd is grinding through the hard surface - most cams/lifters are only induction hardened a few thousands. They heat the cam white hot, then oil quench. But the heating was such a thin amount, you can handle the cam with bare hands right away. Its actually scary to watch the 1st time - someone handling cam that was white hot a moment ago.
**If your roller cam lift rate is so radical that it exceeds the edge of the flat tappet, the tappet will dig into the cam and failure will be almost instant.
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